The present invention relates to a movable part of a wire-dot print head used in an impact printer, and more particularly to a method of producing a movable part comprising a lever to which a print wire is fixed and an armature supported by a plate spring.
As is well known, impact printers have an advantage of being low priced, and is capable of printing on a variety of media, so they are utilized in output devices of data processing systems and various other applications. With the widespread application, their performance is also improved, but still in recent years, even higher printing speed and even higher reliability of printing are demanded.
To answer these demands, in wire-dot print heads of the spring-charge type used in impact printers, it is desired to improve the strength of the movable part comprising the lever to which the print wire is fixed and the armature supported by the plate spring thereby to avoid breakage of the movable part due to fatigue and to ensure a long-time stable operation.
A wire-dot print head of the spring-charge type will be taken as an example for further explanation with reference to FIG. 2.
FIG. 2 is a side view of a wire-dot print head. To clarify the internal structure, the lower half is shown in section.
In the figure, reference numeral 1 denotes a print wire. Reference numeral 2 denotes a lever to which a base part of the print wire 1 is fixed. Reference numeral 3 denotes an armature to a tip part of which a base part of the lever 2 is fixed. A base part of the armature 3 is supported to a tip of a biasing plate spring 4. A base part of the plate spring 4 is fixed to an armature support 5.
Reference numeral 6 denotes a first yoke. Reference numeral 7 denotes a first annular magnetic spacer. Reference numeral 8 denotes a second annular magnetic spacer. Reference numeral 9 denotes an annular permanent magnet. Reference numeral 10 denotes a base on the central part of which a plurality of cores 11 are arranged to form substantially a circle or an ellipse. On the periphery of the base 10, the first yoke 6, the magnetic spacer 7, the second yoke 8, and the permanent magnet 9 are stacked in a predefined order. On the inner side of the first yoke 6, which is disposed at the frontmost position, the armature support 5 is fixed so that the armature 3 extends to confront the core 11.
The movable parts each comprising the print wire 1, the lever 2, the armature 3 and the plate spring 4 are provided in the same number as the cores 11.
Reference numeral 12 denotes a demagnetizing coil fitted on each core 11. Reference numeral 13 denotes a cover mounted on the outside of the first yoke 6. To a tip of the cover 13 formed to protrude at the center of the cover 13, the tips of the print wires 1 are guided and regulated to be in a predefined arrangement.
Reference numeral 14 denotes an ink ribbon. Reference numeral 15 denotes printing media such as printing paper. Reference numeral 16 denotes a platen disposed to confront the guide part of the cover 13, through the ink ribbon 14 and the printing media.
The operation of one movable part of the wire-dot print head of the above structure will be briefly described.
When the demagnetizing coil 12 is not energized, the magnetic flux of the permanent magnet 9 flows through a magnetic circuit comprised of the second yoke 8, the magnetic spacer 7, the first yoke 6, the armature 3, the core 11 and the base 10. As a result, because of the magnetic attracting force generated between the armature 3 and the core 11, the armature 3 is attracted to the core 11, bending the plate spring 4.
At that time, because of the attracting operation, the print wire 1 is displaced, together with the lever 2, toward the base 10, and this displaced position is the initial position for the print wire and the lever 2.
In this state, if the demagnetizing coil 12 is energized, the magnetic flux of the permanent magnet 9 is canceled, and the armature 3 is released from the attracting force of the core 11, and the plate spring 4 restores its shape and the armature 3 is separated from the core 11.
Because of the separating operation of the armature 3, the lever and the print wire 1 are driven and the tip of the print wire 1 projects out of the tip of the guide part of the cover 13, and the projecting tip impacts, through the ink ribbon 14 and the printing media 15, the platen 16, so that ink on the ink ribbon 14 is transferred, as a dot, onto the printing media 15.
After that, because of the repulsion to the impact, the print wire 1 begins returning in the direction opposite to the direction in which it projected, and at the same time, the current to the demagnetizing coil 12 is interrupted. As a result, the magnetic flux of the permanent magnet 9 flows through the above-mentioned magnetic path, and accordingly the armature 3 is again attracted to the core 11. Consequently, the print wire 1 and the lever 2 return to the initial position.
The above is an operation during one cycle of printing operation. In actual printing, each movable part is selectively driven responsive to the print data, and characters or the like formed of dots are printed.
The conventional movable part in the above described wire-dot print head has the following structure. As the lever 2, maraging (martensite aging) steel, Elgiloy (tradename), or the like generally known as a high-strength spring material is used, and as the material of the armature, silicon steel, Permendur or any other high-magnetic flux density material is used, so as to reduce the weight, and the base part of the lever 2 and the tip of the armature 3 are bonded. The method of the bonding normally employed is brazing. Generally, the brazing filler materials have a melting point lower than the materials to be bonded to each other. Elements of the same kind, i.e., elements containing the same atoms, as the materials to be bonded, or elements having affinity with the materials to be bonded are mixed in the brazing filler materials, so that sufficient bond strength can be obtained.
For bonding the base part of the lever 2 and the armature 3, silver brazing is employed. Where a high strength is required, the lever materials and the armature materials are properly selected and the copper brazing is conducted in a non-oxidizing atmosphere.
In the above-described prior art, at the time of impact of the print wire on the platen during printing operation, and at the time of re-attraction by the core of the armature, the movable part comprising the print wire, the lever, and the armature receive impact, breakage and separation of the bonded part between the lever and the armature can occur.
One reason for this is considered to be that as flux is used in the silver brazing, oxidation near the bonded part due to the flux can deteriorate the lever material strength. Moreover, where copper brazing is used for the bonding, the bonded part is easy to be oxidized in wet atmosphere, or in atmosphere containing chlorine, or sulfur, and is easy to corrode, and hence the strength is degraded. This is an obstacle to obtaining a high reliability of a printing head.